Members value of the ScriptableObject type field in the EditorWindow resets to the default value when entering Play Mode

How to reproduce:

1. Open the attached project "Repro.zip"
2. Open Assets/TestSO.asset
3. Enter Play Mode
4. Exit Play Mode
5. Repeat step 3

Expected results: "Copy Value" in the "TestSoEditor" window is 1
Actual results: "Copy Value" in the "TestSoEditor" window is 0

Reproducible with: 2020.3.45f1, 2021.3.18f1, 2022.2.6f1, 2023.1.0b3, 2023.2.0a1

Reproducible on: Windows 10